A technique of measuring an output frequency of a crystal oscillator is known in which an oscillation output of a crystal oscillator is transmitted in a form of a radio wave from a transmission antenna to a temperature tank such that the radio wave is received by a reception antenna provided in the temperature tank, and the output frequency is measured based on an output of the reception antenna.
[Patent Document 1] Japanese Laid-open Patent Publication No. 01-186003
However, it is difficult to detect a state of the crystal oscillator before a transition to an output stop state (for example, clock stop) based on such a measurement result of the output frequency of the crystal oscillator that is obtained according to the conventional technique as described above. Such an output stop of the crystal oscillator may occur suddenly due to abnormality of the crystal oscillator or the like.